Phase 1 Results from a Phase 1/2 Study to Assess the Safety, Tolerability and Recommended Phase 2 Dose (RP2D) of Brentuximab Vedotin Plus Doxorubicin, Vinblastine and Dacarbazine (A+AVD) in Pediatric Patients (Pts) with Advanced Stage Newly Diagnosed Classical Hodgkin Lymphoma (cHL)